Hillary Clinton believes Sen. Bernie Sanders, I-Vt., caused "lasting damage" to the Democratic Party and laid the foundation for "[Donald] Trump's 'Crooked Hillary' campaign," according to excerpts of her new book "What Happened" posted to Twitter by her supporters.

Clinton's book, set for release Sept. 12, recounts her experiences during the 2016 election and stunning loss to Trump. She also shares her experience campaigning against Sanders, whose policies she calls a "pipe dream."

"Throughout the primaries, every time I wanted to hit back against Bernie's attacks, I was told to restrain myself. Noting that his plans didn't add up," Clinton wrote.

The excerpts of Clinton's book provide a candid assessment of Sanders from a candidate who was known to be more cautious when it came to criticism of the socialist senator from Vermont during her failed 2016 White House bid.

She accuses Sanders of resorting to "innuendo and impugning my character" because the two Democrats "agreed on so much."

"Some of his supporters, the so-called Bernie Bros, took to harassing my supporters online. It got ugly and more than a little sexist," Clinton added.

In the excerpts, first posted by a pro-Clinton Twitter account and by a user on a Clinton "super volunteer" Facebook page, Clinton shares how President Obama urged her to not attack Sanders.

"President Obama urged me to grit my teeth and lay off Bernie as much as I could. I felt like I was in a straitjacket," Clinton said.

"When I finally challenged Bernie during a debate to name a single time I changed a position or a vote because of a financial contribution, he couldn't come up with anything," Clinton wrote. "Nonetheless, his attacks caused lasting damage, making it harder to unify progressives in the general election and paving the way for Trump's 'Crooked Hillary' campaign."

Clinton continued her attack on Sanders, charging he "isn't a Democrat" and he did not run in 2016 to ensure Democrats maintained control of the White House.

"That's not a smear, that's what he says," she wrote. "He didn't get into the race to make sure a Democrat won the White House, he got in to disrupt the Democratic Party."

"I am proud to be a Democrat and I wish Bernie were, too," Clinton concludes.

Bernie Sanders has done little to quell speculation that he may mount another bid for the White House in 2020, recently telling CNN "I am not taking it off the table."